1. Field of the Invention
This invention relates to an electrophotographic printing device, and more particularly to an electrophotographic printing device which can selectively print different types of recording papers.
2. Description of the Related Art
A laser printer is known as an electrophotographic printing device, for example. A typical laser printer includes a charging drum or photosensitive drum rotated in a predetermined direction at a preset speed, and a charging section, light exposure section, developing section, image transfer section, fixing section and charge removing section which are arranged around the peripheral portion of the charging drum and in the rotation direction thereof. In the printing process, the surface of the charging drum is electrostatically charged by means of the charging section, and then selectively exposed to a laser beam by means of the light exposing section. An electrostatic latent image (charge pattern) representing characters or patterns is formed on the surface of the charging drum by scanning the laser beam o the charged surface of the charging drum. After the formation of the electrostatic latent image, toner is supplied to the charging drum from the charging section and adhered to those portions of the surface of the charging drum which correspond to the electrostatic latent image, thus developing the electrostatic latent image. After the development, the toner image on the charging drum is transferred to a recording paper by means of the image transfer section. For example, the recording paper is supplied from a paper supply cassette and fed into between the charging drum and charging electrode so as to be set in contact with the toner on the charging drum and the charging electrode. The image transferring process is effected by applying a predetermined image transferring D.C. voltage between the charging drum and the charging electrode under this condition and then separating the recording paper from the charging drum. The surface of the recording paper is charged with the same polarity as the surface of the charging drum by the image transferring D.C. voltage. When the recording paper is separated from the charging drum, the toner on the charging drum is attracted by means of electrostatic forces and adhered to the recording paper. After the image transferring process, the electrostatic charges on the surface of the charging drum are removed by means of the charge removing section, and heat and pressure are applied to the recording paper by means of the fixing section. As a result of the fixing process, the toner image on the recording paper is fixed on the recording paper. After this, the recording paper is discharged from the printer to the outside.
In general, a conventional electrophotographic printing device is so constructed as to selectively supply different types of recording papers. In most cases, a plurality of paper supply cassettes are provided for recording papers of different sizes, for example, A4, B5 and letter sizes, and the recording papers of the different sizes are received in the respective paper supply cassettes. The operator selects a paper supply cassette having recording papers of a desired size received therein and sets the cassette in the electrophotographic printing device prior to the printing process.
Recently, an electrophotographic printing device which can selectively supply sheet of recording paper of postcard size relatively smaller than the conventional paper size has been developed. However, in a case where the postcard is actually used as a recording paper, the printing quality will be deteriorated by an insufficient image transferring process. The insufficient image transferring process is caused by the thickness of the postcard which is larger than that of the standard recording paper for the electrophotographic printing device. In this case, it is difficult to sufficiently charge the surface of the postcard by the image transferring voltage. Further, the insufficient image transferring process may be caused by the quality of the postcard.